Medical Need: Human immunodeficiency virus (HIV), the etiologic agent of acquired immune-deficiency syndrome (AIDS), continues to infect millions of people worldwide. Despite recent improvements in antiretroviral therapy, there remains an urgent need for novel and improved methods of treatment for individuals living with AIDS. Highly active antiretroviral therapy (HAART) has dramatically improved patient outcomes however, the limitations of treatment options include the emergence of resistant viruses and reduced patient compliance as a result of adverse side effects. Disruption of the formation of the six-helical bundle gp41 coiled coil, which is the final step of HIV-1 membrane fusion and infectivity of its host cell, is an example of a novel mechanistic treatment modality. Enfuvirtide (Fuzeon), a fusion inhibitor, was recently approved by the FDA. Goal of Research: Locus Pharmaceuticals' innovative computational technology, which requires only a crystal structure as input, rapidly and accurately identifies the biologically relevant active binding site(s) of a protein and simultaneously designs novel, small molecule antagonists or agonists. This de novo ligand design process has been applied to the gp41 six-helical bundle crystal structure recently published by Kim and coworkers and this has resulted in the identification of a series of low molecular weight, drug-like molecules. These compounds are capable of disrupting HIV-1 gp41 helix bundle formation and provide cell-based cytoprotection in HIV-1 challenge assays. Optimization of this chemical series for biologically relevant physical properties and cell-based cytoprotection assays are the focus of this grant proposal. Specific Aims: Synthesize a select set of analogs in two chemical series and determine the helical bundle disruption (HBD) activity. Identify novel small molecules with enhanced drug-like properties, such as low non-specific protein binding, aqueous solubility, and Caco-2 cell permeability. Determine the activity of molecules in a cell-based cytoprotection assays against several strains the HIV virus.